Lithium ion secondary batteries are the most typical example of non-aqueous electrolyte secondary batteries. Lithium ion secondary batteries have high energy density and can be reduced in size and weight. Usually, they have a structure as described below.
Lithium ion secondary batteries have an electrode assembly in which a positive electrode plate comprising a current collector and a positive electrode active material layer carried on the current collector and a negative electrode plate comprising a current collector and a negative electrode active material layer carried on the current collector are spirally wound with a separator serving as an isolation layer therebetween. This electrode assembly is housed into an outer case with a non-aqueous electrolyte. The separator is made of a material insoluble in non-aqueous electrolytes. For example, microporous films made of a polyolefin resin such as polyethylene resin and polypropylene resin, and polymer films containing polyethylene oxide, polyvinylidene fluoride or polyacrylate are used. The non-aqueous electrolyte is usually a polymer gel electrolyte or non-aqueous electrolyte solution. The polymer gel electrolyte is a polymer electrolyte containing a non-aqueous electrolyte solution.
The non-aqueous electrolyte solution comprises a solute such as a lithium salt dissolved in a non-aqueous solvent. The solutes include, for example, lithium hexafluorophosphate (LiPF6). The non-aqueous solvents include ethylene carbonate, dimethyl carbonate, etc. As the positive electrode active material, lithium-containing transition metal oxides such as lithium cobalt oxide (LiCoO2) are used. The negative electrode active material comprises a material capable of reversibly absorbing and desorbing lithium ions. For example, carbon materials such as graphite are used.
Non-aqueous electrolyte batteries have the advantage of capable of being charged to a higher voltage and having higher energy density. Due to high voltage and high energy density, the non-aqueous electrolyte is often decomposed by oxidation on the positive electrode. On the negative electrode, on the other hand, the non-aqueous electrolyte is often decomposed by reduction because the negative electrode has a very low electrochemical potential. These decomposition reactions tend to occur more readily at high temperatures, and a large amount of gas is generated when the battery is stored at a high temperature of 60 or 85° C.
The non-aqueous electrolyte battery is used as a power source for driving electronic devices such as notebook computers. The temperature inside a notebook computer is usually 45 to 60° C. Under such temperature conditions, the battery is charged at a constant voltage of 4.2 V, and the battery in a charged state is sometimes left for a while. When the battery in such a charged state is stored at high temperatures as just described, gas is more likely to be generated inside the battery as compared to when the battery in an open circuit condition is stored at high temperatures. As a result, the generation of gas during high temperature storage causes an increase in pressure inside the battery. This activates a protection circuit of the battery which shuts down the current, and the battery loses its function as a battery.
Because non-aqueous electrolytes have the problem that the non-aqueous solvent is easily decomposed and gas is easily generated during high temperature storage. In order to cope with this, proposals are made to add an imide salt or a phosphoric acid ester to a non-aqueous electrolyte.
There is proposed a battery in which a quaternary salt of a compound having an asymmetric chemical structure having a conjugated structure containing nitrogen is further incorporated into a non-aqueous electrolyte solution comprising a supporting electrolyte dissolved in a non-aqueous solvent (Patent Document 1). Also proposed is a battery whose non-aqueous electrolyte solution comprises a supporting electrolyte containing lithium ions as cations dissolved in a non-aqueous solvent comprising a cyclic compound to which a surfactant is further added. As an example of the cyclic compound, a cyclic phosphoric acid ester is proposed. As an example of the supporting electrolyte, an imide salt is proposed (Patent Document 2).    Patent Document 1: Japanese Patent No. 3060107    Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-33119.